Type formatters for the LLDB debugger

[mentlerd]

Dear ImGui already contains much appreciated support for debuggers to visualize it's custom types at misc/debuggers.

This PR introduces the same capability for LLDB, the choice of debugger of Xcode and macOS. Here is a quick sample of a before/after:

ImVec2 vec2(1, 2);
ImVec4 vec4(1, 2, 3, 4);
ImRect rect(1, 2, 3, 4);

ImVector<int> vec;
vec.push_back(1);
vec.push_back(2);
vec.push_back(3);
vec.push_back(4);

ImSpan<int> span(vec.begin()+1, vec.begin()+3);

printf("Breakpoint here!");

Before	After

Admittedly the amount of code to achieve this might be scary. LLDB's scripting integration was built with more complex data structures in mind, thus doing simple things unfortunately takes quite a bit of legwork.

Regardless I thought this might be useful for people who use either LLDB or Xcode to work with Dear ImGui.

[ocornut]

Hello,
Thanks for your PR. Do any OSX user want to review this too?

Feedback
(A) file should probably be called imgui_lldb.py or something else than just imgui.py

(B) this is not safe:

child = 1 if flags & 0x01000000 else 0
popup = 1 if flags & 0x04000000 else 0

As we don't provide binary compatibility. If you can't access the C++ names then best to remove those.

[mentlerd]

Done and done! I took the formatting and flag values from the already available .natvis file in the same folder.

[ocornut]

Thank you! Merged as 087fbf0 + amends 46e6382.

Would you be able to provide the equivalent patch for ImStrv in the string_view branch?
https://github.com/ocornut/imgui/tree/features/string_view/misc/debuggers

.natvis:

<!-- String view (non zero-terminated begin/end pair) -->
<Type Name="ImStrv">
  <DisplayString>{Begin,[End-Begin]s8} ({End-Begin,d})</DisplayString>
  <StringView>Begin,[End-Begin]s8</StringView>
</Type>